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MOSFETs versus BJT in discrete designs
DMartens:
When it comes to integrated circuits, MOSFET technology have overtaken BJT technology for a long time now. Most new ICs are implemented using some form of FET technology, whether (A)LS, HCT, etc.
However, I notice that when it comes to designs with discrete components, most electronics engineers will still go for BJTs instead of MOSFETs. Why is that? I doubt price / availability is the answer, nor speed / performance / choice etc. So what are some of the reasons?
T3sl4co1l:
Well, address confirmation bias first.
From what era are you mostly seeing these designs?
Hint: even if it's current, posted on the web, etc., a vast number of designs have heritage back to the 70s, 60s even. Almost anywhere you see 2N2222(A) (partial points for PN2222A), you can bet someone is either copying directly, mutating with minor changes, or effectively remixing such old content. (Blindly reusing a long-irrelevant part like 2N2222, I would argue counts as remixing: it's the meme among BJTs.)
There is a strong heritage, or bias, for BJTs in audio designs as well, which is I suspect better justified overall, but audio is also one of those subsets where practitioners regularly go out of their way to do things differently. Often "different" as in "wrong", as judged in strict engineering terms. There are a number of designs with MOSFETs involved (JFETs too, even), but this can be as often as not read in terms of that forced difference, as it is real engineering merit.
And then, "everything else" probably includes what, largely power and digital? And BJTs have been largely out of power electronics for a very long time, with a few surviving niches (mainly extremely cost-sensitive, modest efficiency, medium to low power applications, mostly lighting and small power adaptors), while logic I would say uses a fairly even mix of MOSFETs and BJTs, usually I would say depending on the user's experience with them, and knowledge of the circuit they're working on.
For that matter, TVs, CRTs, used BJTs for a very long time as well, even as MOSFETs became better generally; this is only partially by accident, as MOSFETs didn't improve quite as quickly as needed, by the time CRTs were obsoleted (namely, that horizontal output transistors require quite high voltages (typically 1500Vcbo), and quite high currents for the largest high-res monitors (10A+ peak), which wasn't feasible with planar MOSFETs, but is with modern SuperJunction types, but they came along, eh, 5-10 years too late to offer, eh, merely marginal improvements to CRT operation, if any?). Meanwhile, CRTs, especially high resolution, Trinitrons and such, have quite complex circuitry where every quirk of every component matters, and I would expect it would be challenging to preserve the highly-optimized linearity of a BJT line output circuit while migrating it to use a SuperJunction MOSFET. (That said, it's not that they didn't touch them at all: S-correction was a typical application for modestly-rated MOSFETs.)
Probably some things about automotive too; Darlington transistors probably stuck around a long time? Internally protected MOSFETs are all the rage these days, but I don't know what a real market breakdown is like these days.
In any case, LED control remains a useful application; BJTs are easy to bias, fine at dissipating power at up to modest voltages (10s, maybe low 100s), and so are good for CCSs (indication or lighting).
BJTs also excellent for certain linear, switching and oscillator applications. ZVS "Royer" (actually Baxandall) oscillator is particularly appealing with BJTs, I mean it's quite good with MOSFETs too, but BJTs are (were?) a common sight for CCFL supplies for example.
As for actual engineering merits, independent of whether any particular designer is aware of them, or using them in furtherance of an actual spec (like bandwidth or efficiency) or for optimization (whether a specified parameter or yak-shaving): BJTs hold the distinction of a higher linear-mode figure-of-merit, defined as gm / (Cin + Cout) for example. That is, the input takes less voltage, and the output has more gain, puff for puff, than MOSFETs do.
There is also a practical side, in that small MOSFETs simply don't exist; RF types disappeared long ago, and I mean, good luck finding a BFR92 today too (which, looks like Infineon is still making some variant thereof and a few relatives, but everyone else's is obsolete; but then, I dare you go looking for the complement BFT92..!), but among MOSFETs the similarly sized parts simply haven't existed for decades. 2N7002 remains a staple despite its age, but its ~30pF junction is massive in comparison to the ~4pF of a MMBT3904, on top of a 3-5V working Vgs(on) range.
Speaking of complements, MOSFETs there simply is no such thing, only hand-wavey use-alikes. BSS84 is often quoted as a complement to 2N7002, but it doesn't really mean much; PMOS is either ~2.5 times more capacitance (same Rds(on)), 2.5 times more Rds(on) (same Ciss etc.), or somewhere inbetween; and with Vgs(th) individually varying a volt or so, matching pairs doesn't really mean anything in terms of that parameter either. Whereas BJTs, PNP and NPN are remarkably similar, PNP generally being about 10% poorer performance, something like that, and there are some notable differences in curves especially hFE at low Ic, and fT, but complementary pairs are definitely a meaningful thing among them.
Finally, if you want to build out a modest-scale discrete circuit, and want to push the performance envelope say in terms of functionality and efficiency per supply current consumption, you will be using BJTs, you will be taking advantage of complementary pairs, and other tricks (as well as you can, given the limited matching, especially thermally so, of discrete devices), to reduce component count (layout space, assembly cost, additional load capacitance, increased supply consumption?), and you'll still end up with a laughably inferior circuit to even a fairly simple IC -- if one existed for your particular application, which I'm guessing given the effort required of this hypothetical scenario, doesn't exist.
An example would be this current-limiting electronic fuse design I made a bit ago,
https://www.seventransistorlabs.com/Images/LimitingFuseSch.png
which boasts pretty low supply consumption in most any state, and a little more during active limiting (peak current up to a few mA during gate voltage slewing). Speed could be improved with RF transistor types, but nothing's going to come anywhere close to the price of BC847/57 pairs, and the limit is mostly diff pair + VAS bias + gate capacitance, and is fast enough (10-20µs response time from load shorting to nominal current limiting).
Incidentally, the MOSFET is both optimal for steady-state efficiency (Rds(on) performs better than Vce(sat); no Ib requirement), and the modest voltage swing makes it easy to compensate (it's a voltage gain node so C5 doesn't need to be large to do its job). A BJT circuit would have to resolve not only saturation (a sort of Baker-clamped Darlington to maintain low Vce(sat) while being miserly with Ib?) but probably need the error amp and compensation done in a current-mode scheme (since the voltage change in Vbe will be quite small), or an additional stage to get a transconductance characteristic, either way taking up many more transistors. (Which can of course be justified, and readily integrated, on an IC, hence all the bipolar LDOs with good specs available today, particularly low Iq vs. Iout and over Vin (including in dropout), and compensation into ceramic capacitors.)
Why bother? There are E-fuse parts readily available today; but they're largely (entirely??) integrated, no external switch. There are some load switch, hot-swap, wired-OR, etc. controllers that use an external MOSFET, but few with active current limiting AND adjustable SOA. And, noteworthy, no such circuit can be made of IC building blocks -- even an op-amp of comparable GBW (100s kHz) would take as much Icc, let alone the comparator(s) or timer or whatever to handle the SOA limit function, and the latch on/off and autorestart functions.
And despite all this, it was still just a design exercise; I didn't have (and still don't) an intended application or use-case for this, and haven't produced and sold any units (I probably should just make a few some day, and put them up somewhere and see how that goes, but, meh?). I don't expect that I can sell many at a price point that would be in any way economical for me to do so, and it's just not so fancy a product to justify such a price.
But if you have additional restrictions, or limitations, like an import-restricted country where only basic BJTs and whatnot are available, well, gotta do what you gotta do of course, and that will include compromises in assembly cost, performance, etc.
Tim
coppercone2:
well mosfet are creepy transistors if you probe them it might change states for a while, it feels like dealing with a sticky icky card house or like duct flaps etc (the gate between your dryer and the outside that might get stuck)
creepy icky transisticy
SiliconWizard:
--- Quote from: DMartens on November 20, 2023, 04:23:06 am ---However, I notice that when it comes to designs with discrete components, most electronics engineers will still go for BJTs instead of MOSFETs.
--- End quote ---
First off, "most", I don't know?
Other than that, many (I would not say "most") have a harder time understanding the behavior of MOSFETs, in particular when dealing with their linear region. BJTs look simpler in comparison. On the surface.
Of course, there are applications for which BJTs are more appropriate than MOSFETs. But when that's not the case, it's often either due to design habits, or a lack of understanding. So, the comfort zone.
As an illustration, to stay on the surface of things (getting deeper can get hairy fast), how many can give the simplified equation giving the collector current as a function of the base current of a BJT? Now, how many can give the simplified equation of the drain current vs. Vgs and Vds of a MOSFET? What about the saturation region?
So while I wouldn't say "most", as it sounds kinda bold, IME outside of engineers having specialized in microelectronics, relatively few properly understand how MOSFETs work, and when they use them, they tend to stick to using them as switches and at most care about driving them with enough voltage and with enough peak current to overcome the gate capacitance. Beyond that, you'll probably look at 20% of EEs or something like this.
magic:
Discrete MOSFETs have some characteristics which make them at least a little annoying in analog design.
Higher Vgs than typical Vbe of bipolars, which can be problematic in low voltage circuits needing to work close to rails.
Higher production spread of Vgs, making matching harder.
Plentiful 1/f noise (this also applies to CMOS ICs, although some of the newer ones are not as bad as some of the older ones).
Very, very rarely rated for SOA at DC.
OTOH, the lack of base current is a welcome property which makes FETs useful in applications like precision current sources or active loads.
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